The specific aims of this study are: (1) to measure baseline bone turnover by serum and urine biochemical markers in men over age 65 years with low femoral neck bone density; (2) to measure biochemical markers of bone turnover after 9 weeks treatment with 17-B estradiol; (3) to examine the relationship between baseline bone turnover and bone mass at the femoral neck, lumbar spine and total body in older men; and, (4) to examine the response of calcium regulating and sex hormones to low dose estrogen replacement therapy in older men. Screening for eligible patients is continuing.